1. Field of Invention
The present invention relates to a crane, in particular a tower crane, with a boom rotatable about an upright slewing gear axis and an out-of-service brake which permits and slows down the rotary movements of the boom in the out-of-service condition.
2. Background and Related Art
In tower cranes, but also in other crane types, the boom is rotatable about an upright slewing gear axis, wherein a slewing gear provided for this purpose can include a slewing drive for example in the form of an electric motor whose drive movement is converted into a rotary movement of the boom via a slewing gear transmission for example in the form of a planetary transmission. In so-called top-slewing cranes, the boom is rotated relative to the tower carrying the boom, while in so-called bottom-slewing cranes the entire tower along with the boom mounted thereon is rotated relative to the undercarriage or the supporting base.
In crane operation, the rotary movements are controlled by correspondingly actuating the slewing drive, wherein for braking and also for rotatorily fixing in a certain rotary position a slewing gear brake is provided. For safety reasons, such slewing gear brakes usually can be formed such that the brake is biased into its braking operating position for example by a corresponding spring means and can be vented by an actuator, in order to enable the rotatability.
In non-operation or in the out-of-service condition, when the crane is shut off, it is desirable however that the crane is able to rotate, in order to align itself in the wind into the most favorable rotary position for the respective wind direction. Since for example tower cranes due to their ballast usually are very much more stable against tilting movements in the boom plane than against tilting movements transversely to the boom planes going vertically through the boom, the crane should align itself such in strong wind that the wind comes from the rear and the boom is aligned with the wind as parallel as possible to the wind direction, since otherwise the crane would threaten to tip over or the crane would have to be ballasted in addition. To allow such automatic alignment in the wind, a wind-release device is associated to the service brake or slewing gear brake, which vents the brake usually biased into its braking position, when the crane is out of operation. This “end-of-work” position of the slewing gear brake can be adjusted by means of a manually actuatable control lever, but possibly also by a motorized venting drive, which can move the brake actuator into a locked non-braking position before shutting off the crane. Such wind-release device for the slewing gear brake of a tower crane is shown for example in the document EP 14 22 188 B1.
Under unfavorable wind conditions, however, the free rotatability of the crane in the out-of-service condition can lead to instabilities of the crane due to self-rotation. For example, when the crane is positioned between two buildings and only the boom or only the counter-boom is exposed to the wind, only the boom or the counter-boom each is unilaterally hit by the wind, whereby the crane can be put into a faster and faster rotation, since the crane does not stop when the boom has rotated out of the wind or before the counter-boom gets into the wind. As a result the boom and the counter-boom alternately can get into the wind, so that an amplification of this cyclic wind application can lead to an autorotation of the crane, which rotates the crane too fast and makes it tip over.
To avoid such unwanted autorotation, it has been proposed already to make the slewing gear rotate not entirely unbraked in the out-of-service condition, but to assign an additional brake to the slewing gear, which permits the rotary movement of the crane under wind, but slightly brakes the same, in order to mitigate the aforementioned autorotation problem. For example, it has been considered to provide a light-weight out-of-service brake at the output of the slewing gear transmission, which applies a limited braking torque against the crane rotation, which is smaller than the torque generated by wind application, so that the crane still is able to align itself in the wind, but only is able to rotate with a small speed of rotation.
Such an additional brake, however, is difficult to design in terms of the braking torque, in order to be equally suitable both for different wind conditions and for different crane positions. For example, with still moderate wind too high a braking torque can lead to the fact that the crane is not aligned properly, whereas with very unfavorable wind conditions with high wind velocities the same braking torque is not able to sufficiently inhibit the autorotation. In tower cranes with luffable boom, the luffing position in which the crane has been shut off also can have an influence on the required braking torque.